I have several cameras available here, for which darktable does not
offer dedicated base curves. And because we are talking about a number
of cameras (more might come in the future), I would like to create a
standardized structure.
https://www.darktable.org/2013/10/about-basecurves/
The base is a simple repro setup as shown in the picture:
https://www.bilddateien.de/blog/bilder/2018-12-23/20181224_n850005.jpg
Half of the screen table is covered with black cardboard. Light comes
from below from the flash. In front of the lens there is a gray filter
103 8x to reduce the influence of stray light in an already dark room.
The camera height and lens are adjusted so that the edge is very blurred
to allow a smooth transition from black to white - this may be improved
further in future.
This all works quite well.
The problem: while I get RGB values of 1...5 in absolute darkness (cover
cap on the lens) in darktable with the pipette for my oldest camera
(D700), the lowest values with this setup are at RGB 12...15. This seems
too high to me.
In the waveform histogram it looks like this:
for a picture like this:
If I expose a bit darker (e.g. by reducing the flash output), then white
becomes too dark (<255) without the lowest values moving significantly
closer to black (=0).
To prevent scattered light from the flash in the surrounding area, I
placed it in a bucket that at the top is even with the matte screen.
This did not result in any change.
Then I covered the whole construction with a dark blanket - also no change.
Two questions:
- is a basic value of 15 as "black" really too high for a useful
determination of the base curve?
- if yes: does anyone have an idea how to improve the setup?
--
regards
Bernhard
https://www.bilddateien.de
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